DESCRIPTION (Investigators Abstract) The purpose of the proposed study is to examine the long-term structural and functional effects of cryotherapy as a treatment for severe retinopathy of prematurity (ROP). In the initial phase of the study, eyes with 5 contiguous or 8 total clock hours of stage 3 plus ROP, in 291 infants with birth weights less than 1251 g, were randomized to receive cryotherapy or no cryotherapy. Follow-up to age 12 months indicated that cryotherapy reduced the incidence of unfavorable structural outcome by 45.8 percent and the incidence of unfavorable visual acuity outcome by 37.8 percent. Based on the preliminary data, follow- up to age 12 months during Phase II of the study indicated that cryotherapy reduced the incidence of unfavorable resolution acuity outcome by 21.4 percent and by 19.7 percent when based on measurement of recognition acuity. The present study proposes to follow patients in the randomized group and a relatively small subgroup (67) of potentially high risk patients from the Natural History study through age 10 years, to determine whether the reduction in the benefit of cryotherapy seen between ages 12 and 42 months continues into mid-childhood. Evaluation of the effects of cryotherapy will be based on examination findings in the posterior pole of the eye and on assessment of visual acuity, as in Phase II. In addition, a more complete evaluation of visual function will be conducted at age 9 years, through assessment of monocular contrast sensitivity, monocular color vision, and stereopsis, and at age 10 years, through measurement of the extent of the visual field by Goldmann perimetry. A simultaneous study is proposed for another subgroup of 539 children with a birth weight less than 1000 grams who were enrolled in the Natural History Study, to determine the relationship between ocular structure and visual function at 9 and 10 years old and the severity of ROP during early infancy. Developmental assessment in the randomized population and this Natural History subgroup at 10 years of age will permit evaluation of the influence of visual defects on developmental outcome, and will provide crucial information to estimate the cost benefit ratio of treatments to reduce visual loss in extremely low birth weight infants.